Mirror micromechanical structure and related manufacturing process

ABSTRACT

A mirror micromechanical structure has a mobile mass carrying a mirror element. The mass is drivable in rotation for reflecting an incident light beam with a desired angular range. The mobile mass is suspended above a cavity obtained in a supporting body. The cavity is shaped so that the supporting body does not hinder the reflected light beam within the desired angular range. In particular, the cavity extends as far as a first side edge wall of the supporting body of the mirror micromechanical structure. The cavity is open towards, and in communication with, the outside of the mirror micromechanical structure at the first side edge wall.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application from U.S. application forpatent Ser. No. 14/151,471 filed Jan. 9, 2014, which claims priorityfrom Italian Application for Patent No. TO2013A000031 filed Jan. 14,2013, the disclosures of which are incorporated by reference.

TECHNICAL FIELD

The present invention relates to a mirror micromechanical structure andto a related manufacturing process.

BACKGROUND

Mirror micromechanical structures are known, which are made, at least inpart, of semiconductor materials and using the MEMS(microelectromechanical systems) technology.

These micromechanical structures are integrated in portable apparatuses,such as, for example, portable computers, laptops, notebooks (includingultra-thin notebooks), PDAs, tablets, and smartphones, for opticaloperations, in particular for directing in desired patterns lightradiation beams generated by a light source.

Thanks to the reduced dimensions, these structures enable stringentrequirements to be met as regards occupation of space, in terms of areaand thickness.

For example, mirror micromechanical structures are used in miniaturizedprojector modules (the so-called “picoprojectors”), which are able toproject images at a distance or to generate desired patterns of light.

In combination with an image-capturing module, a projector module ofthis kind enables, for example, implementation of a three-dimensional(3D) photographic camera or video camera for forming three-dimensionalimages.

The aforesaid mirror micromechanical structures generally include: amirror element, obtained from a body of semiconductor material in such away as to be movable, for example with a tilting or rotation movement,to direct the incident light beam as desired; and a supporting element,which is also obtained starting from a body of semiconductor material,is coupled to the mirror element, and has supporting and handlingfunctions. A cavity is made in the supporting element, underneath, andin a position corresponding to, the mirror element, in such a way as toenable freedom of movement for tilting or rotation thereof.

In particular, applications are known in which the mirrormicromechanical structure is required to generate a reflection patternwith an extensive field of view (FOV), i.e., a reflection of theincident light beam over a wide angular range.

For example, FIG. 1 a is a schematic illustration of an opticalprojection system, designated as a whole by 1, in which the mirrormicromechanical structure 2 is used for reflecting, with a desiredangle, an incident light beam, designated by B, coming from a lightsource 3, for example, a coherent light source of a laser type.

In particular, the mirror micromechanical structure 2, including themirror element, here designated by 4, and the supporting element, heredesignated by 5, in which the cavity 6 is obtained, is mounted in such away that the mirror element 4 is set, at rest, at a wide inclinationangle α with respect to the incident light beam B (the inclination angleα being defined as the angle between the direction of the incident lightbeam B and the normal to the surface of the mirror element 4). Thisinclination angle may be comprised between 40° and 50°, for example 45°,and evidently corresponds also to the angle at which the incident lightbeam is reflected by the mirror element 4.

FIGS. 1 b and 1 c show a respective operating condition of the opticalsystem 1, in which the mirror element 4 is rotated through a negativerotation angle θ (causing, that is, a reduction in the inclination angleα), and, respectively, a positive rotation angle θ (causing, that is, anincrease in the inclination angle α), with respect to the restingcondition.

It will be noted that the solution described is affected by an importantlimitation as regards the field of view (FOV) that can be achieved,which cannot guarantee the desired optical performance, at least ingiven operating conditions.

As shown schematically in FIG. 2, in fact, for positive inclinations ofthe mirror element 4, which entail values of the inclination angle αgreater than a given threshold, a phenomenon of at least partialshadowing or clipping of the reflected light beam may arise, therebyonly a part of the reflected light beam may effectively be transmittedtowards the outside of the mirror micromechanical structure 2, forexample for generation of a desired scanning pattern on an outersurface.

The specific value of this threshold depends on the particular assemblyof the mirror micromechanical structure 2. In any case, there is amechanical rotation angle θ of the mirror element 4 such that thereflected light beam can be at least partially shadowed.

In the example illustrated, this phenomenon is highlighted for apositive rotation angle θ of 20° with respect to the resting condition.

The phenomenon described entails an undesirable deterioration of theperformance of the optical system 1. In particular, the optical system 1may be unable to achieve the desired performance as regards the field ofview FOV.

There is a need in the art to solve, at least in part, this problemafflicting mirror micromechanical structures of a known type.

SUMMARY

According to the present invention, a mirror micromechanical structureand a corresponding manufacturing process are consequently provided.

In an embodiment, a mirror micromechanical structure, comprises: amobile mass which carries a mirror element and is configured to bedriven in rotation for reflecting an incident light beam with a desiredangular range (FOV); said mobile mass suspended above a cavity providedin a supporting body including semiconductor material, wherein saidcavity is so shaped that said supporting body does not hinder the lightbeam reflected by said mirror element within said desired angular range(FOV).

In an embodiment, an optical device comprises: a mirror micromechanicalstructure having: a mobile mass which carries a mirror element and isconfigured to be driven in rotation for reflecting an incident lightbeam with a desired angular range (FOV); said mobile mass suspendedabove a cavity provided in a supporting body including semiconductormaterial, wherein said cavity is so shaped that said supporting bodydoes not hinder the light beam reflected by said mirror element withinsaid desired angular range (FOV).

In an embodiment, a process for manufacturing a mirror micromechanicalstructure comprises: forming a mobile mass which carries a mirrorelement and is drivable in rotation for reflecting an incident lightbeam within a desired angular range (FOV); and forming a cavity which isdesigned to be set underneath said mobile mass in a supporting bodydesigned to be coupled to said mobile mass; wherein forming the cavitycomprises: shaping said cavity so that said supporting body does nothinder the reflected light beam within said desired angular range (FOV).

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, preferredembodiments thereof are now described, purely by way of non-limitingexample and with reference to the attached drawings, wherein:

FIGS. 1 a-1 c are schematic views of an optical system including amirror micromechanical structure, in different operating conditions;

FIG. 2 is a schematic view of the optical system, in yet a differentoperating condition;

FIG. 3 is a perspective plan view of a mirror micromechanical structureaccording to an aspect of the present solution;

FIGS. 4 and 5 are sections of the micromechanical structure of FIG. 3,taken along respective lines of section IV-IV and V-V shown in FIG. 3;

FIG. 6 is a schematic view of the micromechanical structure of FIG. 3,in an operating condition;

FIG. 7 is a perspective plan view of a wafer of semiconductor materialincluding mirror micromechanical structures, according to one embodimentof the present solution;

FIG. 8 is a top plan view of a wafer of semiconductor material includingmirror micromechanical structures, according to a different embodimentof the present solution;

FIG. 9 is a cross-sectional view of the wafer of FIG. 8, taken along theline of section IX-IX;

FIG. 10 is a schematic representation of an optical device, according toan aspect of the present solution; and

FIG. 11 is a perspective view of a variant of the mirror micromechanicalstructure.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGS. 3-5 show, in respective views, a possible embodiment of a mirrormicromechanical structure 10, made using MEMS techniques proper to thesemiconductor industry.

In detail, the mirror micromechanical structure 10 comprises: a firstbody of semiconductor material, for example silicon, here designated by11, in which a mobile mass 12 is provided, for example by removal ofmaterial with chemical etching; the mobile mass 12 has a main extensionin a horizontal plane xy and a thickness that is substantiallynegligible with respect to its extension in the horizontal plane xyalong a vertical axis z, orthogonal to the same horizontal plane xy. Themobile mass 12 is surrounded by a trench 13, which is opened in thefirst body 11 due to the same chemical etching.

The mirror micromechanical structure 10 further comprises a second bodyof semiconductor material, for example silicon, here designated by 14,coupled to the first body 11, in which a cavity, or opening, 15 isprovided, for example by removal of material with deep dry chemicaletching.

The cavity 15 has a certain height in the vertical direction z, smallerthan an overall thickness of the second body 14 (for example, equal toapproximately half of this thickness), so that the second body 14defines a base or bottom surface of the cavity 15.

The mobile mass 12, as described hereinafter, is suspended above thecavity 15, and in a resting condition is substantially parallel to, andfaces, the bottom surface of the cavity 15 and the horizontal plane xy.

The mobile mass 12 has a central portion 12 a, for example circular inplan view (in the horizontal plane xy), set on which is a mirror layer16, constituted by a material with high reflectivity in regard to thelight radiation to be projected, such as for example aluminum or gold.The mobile mass 12 also has end portions 12 b, 12 c, having an elongatedshape and extending on opposite sides with respect to the mobile mass 12along a first horizontal axis x of the horizontal plane xy.

The mobile mass 12 is coupled, at the end portions 12 b, 12 c, toanchorages 18, fixed with respect to the second body 14, by means ofelastic elements 19, of a torsional type, which enable rotation thereofout of the horizontal plane xy.

The elastic elements 19 have a longitudinal extension along the firsthorizontal axis x, within respective recesses 20 inside the end portions12 b, 12 c of the mobile mass 12, and are very thin, i.e., having alength along the aforesaid first horizontal axis x that is much greaterthan the corresponding width (along a second horizontal axis y of thehorizontal plane xy, which forms with the first horizontal axis x andthe vertical axis z three Cartesian axes) and greater than thecorresponding thickness along the same vertical axis z.

The elastic elements 19 moreover define, in their direction of extensionand alignment, an axis of rotation A for the mobile mass 12, passingthrough the geometrical center O of the central portion 12 a of the samemobile mass 12.

The aforesaid end portions 12 b, 12 c moreover carry in a fixed waymobile electrodes 22, shaped like fingers, extending in the horizontalplane xy on opposite sides of the same end portions 12 b, 12 c along thesecond horizontal axis y and within the trench 13.

The mirror micromechanical structure 10 further comprises a fixedportion 23, obtained in the first body 11 and fixed with respect to thesecond body 14, separated from the mobile mass 12 by the trench 13. Thefixed portion 23 carries fixed electrodes 24, which also have afinger-like conformation and a longitudinal extension along the secondhorizontal axis y within the trench 13, in a position facing andcomb-fingered with the mobile electrodes 22.

First contact pads 25 a and second contact pads 25 b are carried byrespective top surfaces of the fixed portion 23 and of the anchorages18, for electrical biasing, respectively, of the fixed electrodes 24 andof the mobile electrodes 22.

In use, application (in a known way, here not illustrated) of adifference of potential between the mobile electrodes 22 and the fixedelectrodes 24 causes torsion of the elastic elements 19 and rotation ofthe mobile mass 12 (and of the associated mirror layer 16) out of thehorizontal plane xy about the axis of rotation A, according to thedesired movement so as to reflect an incident light beam towards theoutside of the mirror micromechanical structure 10.

Again in a known manner, the mobile mass 12 may be rotationally drivenwith an oscillatory movement at its mechanical resonance frequency inorder to maximize the extent of its movement, given a same electricalbiasing.

According to one aspect of the present embodiment, the cavity 15 isshaped in such a way that the second body 14 in which it is obtaineddoes not hinder the light beam reflected by the mirror layer 16 set onthe mobile mass 12, without creating even partial clipping or shadowingof the reflected light beam.

In particular, the cavity 15 extends in the second body 14 along thesecond horizontal axis y (i.e., in a direction transverse to the axis ofrotation A), in such a way as to reach a first edge wall 14 a thatdelimits, parallel to the first horizontal axis x, the second body 14and the entire mirror micromechanical structure 10.

On the opposite side of the axis of rotation A, the cavity 15 is insteaddelimited by a side wall, defined by the fixed portion 23 of the mirrormicromechanical structure 10 and by the underlying portion of the secondbody 14. On the same side of the axis of rotation A, the mirrormicromechanical structure 10 has a second edge wall 14 b, which iscontinuous and uninterrupted throughout the thickness of the second body14.

The cavity 15 hence has an open portion at the aforesaid first edge wall14 a, being open towards the outside of the mirror micromechanicalstructure 10 and in fluid communication with the outside of the mirrormicromechanical structure 10.

The recess 13 that surrounds the mobile mass 12 in the first body 11also extends in the horizontal plane xy parallel to the cavity 15 and ina way corresponding thereto.

Moreover, in the example illustrated, the cavity 15 has, at the firstedge wall 14 a a first width L₁ (measured along the first horizontalaxis x), which is greater than a second width L₂ that the same cavity 15has underneath the central portion 12 a of the mobile mass 12 (thesecond width L₂ basically corresponding to the diameter of the centralportion 12 a of the mobile mass 12).

The process for manufacturing the mirror micromechanical structure 10may envisage: first, the formation of the cavity 15 inside the secondbody 14; and then coupling, via bonding, of the first body 11 on thesecond body 14, provided with the cavity 15, and subsequent formation bychemical etching of the mobile mass 12.

As illustrated schematically in FIG. 6, the conformation of the cavity15, with lateral opening towards the outside of the mirrormicromechanical structure 10, at the first edge wall 14 a, is such thatthe light beams reflected by the mirror layer 16 carried by the mobilemass 12 do not undergo any shadowing or clipping, even for wide anglesof inclination α of the incident light beam, thus ensuring an extensivefield of view FOV for the resulting optical system.

In particular, FIG. 6 shows the entire field of view FOV that can beobtained, in the example approximately 80°, as a function of a positionof negative maximum inclination and of a position of positive maximuminclination of the mobile mass 12, for example with a rotation angle θ,respectively, of between −15° and −25°, in the example illustratedapproximately −20°, and of between +15° and +25°, in the exampleillustrated approximately +20°, with respect to the resting position.

For instance, the mobile mass 12 may even have a rotation angle θ equalto 25° (starting from an initial position at 40° with respect to theincident light beam), or equal to 20° (starting from an initial positionat 50°), without any shadowing of the reflected light beam.

FIG. 7 (which is simplified for reasons of clarity of illustration)regards a final step of the process for manufacturing the mirrormicromechanical structure 10, before which previous process steps havebeen carried out to form, amongst other elements, the cavity 15 and themobile mass 12.

The mirror micromechanical structure 10 is the result of a dicingoperation, along scribe lines SL, SL′ parallel to the first horizontalaxis x.

One aspect of the present solution envisages, in particular, that thedicing operation is carried out via a laser, in order to prevent anyresidue of particles or water, which might subsequently jeopardize theoptical performance of the system.

In the embodiment shown in FIG. 7, the mirror micromechanical structures10 are, in particular, obtained starting from a SOI(silicon-on-insulator) structure, with a first wafer 29 (equivalent tothe first body 11) constituted by the active layer of the SOI structureand a second wafer 30 constituted by the deep layer of the SOIstructure, coupled at the back to a further supporting wafer 28 viabonding (in this case, the assembly of the wafers 28 and 30 constitutethe second body 14). The manufacturing process is thus simplified in sofar as the cavity 15 may be obtained by etching the second wafer 30 fromthe back, and definition of the mobile mass 12 may be obtained viaselective etching of the semiconductor material of the first wafer 29.

Inside the wafers 28, 29, 30, appropriately coupled to one another, aplurality of mirror micromechanical structures 10 have thus beenpreviously obtained, and the dicing operation for their separation iscarried out as back-end process stage.

Adjacent mirror micromechanical structures 10 are symmetrical andspecular with respect to the scribe lines SL, SL′.

In the embodiment illustrated in FIG. 7, mirror micromechanicalstructures 10, adjacent to one another, have cavities 15 communicatingand facing one another along first scribe lines S_(L).

After the dicing operation, which leads to opening of the cavity 15towards the outside, the first edge wall 14 a of each mirrormicromechanical structure 10 is moreover defined along the same firstscribe lines S_(L). Simultaneously defined along second scribe linesS_(L)′, which alternate with the first scribe lines S_(L) along thesecond horizontal axis y, are the second edge walls 14 b of the mirrorstructures 10.

FIG. 8 shows a different embodiment, in which mirror micromechanicalstructures 10, adjacent to one another, are set alongside one anotheralong the scribe lines, here designated as a whole by SL, without therespective cavities 15 communicating with one another.

In this case, the dicing operation carried out along each scribe lineS_(L) leads to opening of a respective cavity 15 for a first mirrormicromechanical structure 10, and at the same time to definition of thesecond edge wall 14 b of a second mirror micromechanical structure 10adjacent thereto, as highlighted also in the cross-sectional view ofFIG. 9 (relating to the result of the step of dicing along the scribelines SL).

The above embodiment may prove advantageous in given applications in sofar as the assembly is, prior to dicing, sturdier and stronger from themechanical standpoint, hence being less subject to any possible failureor damage.

The advantages of the solution described emerge clearly from theforegoing description.

It is in any case emphasized that it is possible to obtain extremelycompact dimensions of the final micromechanical structure, at the sametime ensuring high optical performance, in particular as regards theextension of the field of view FOV.

It is, for example, possible to obtain angular scanning ranges of thereflected light beam comprised between 80° and 100°, without theintervening phenomena of clipping or shadowing of the reflected lightbeam.

The manufacturing process does not require additional process steps ascompared to traditional solutions in so far as the lateral opening ofthe cavity 15 is obtained by means of the same dicing operations asthose that lead to separation of the various dice of the mirrormicromechanical structures.

The manufacturing process is particularly advantageous in the case whereSOI structures are used.

The aforesaid characteristics thus render use of the mirrormicromechanical structure particularly advantageous in optical systemsintegrated inside portable devices.

By way of example, FIG. 10 is a schematic illustration of the operationof the mirror micromechanical structure 10 in a projector module 30 of adevice for formation of three-dimensional images 32 in a portableapparatus 33 (such as, for example, a notebook portable computer), whichfurther comprises an image-capturing module 34, for example operating inthe infrared field, and a processing module 35.

In particular, the mirror micromechanical structure 10 is driven so asto project the light beam generated by a light source 36, for example asource of coherent light of a laser type, according to a scanningpattern, within the field of view FOV, which also includes an object 37of which the three-dimensional image is to be reconstructed.

Conveniently, the mobile mass 12 of the mirror micromechanical structure10 is oriented, in a condition of rest, at a wide angle of inclinationwith respect to the incident light beam generated by the light source36, for example between 40° and 50°.

The projector module 36 comprises an appropriate electronic control anddriving circuit 38, which is able to supply appropriate driving signalsboth to the light source 36 and to the mirror micromechanical structure10 so as to vary the orientation thereof according to the scanningpattern desired for the reflected light beam.

Advantageously, also the electronic circuit 38 may be obtained in anintegrated way with semiconductor techniques, possibly in the same dieas the one in which the mirror micromechanical structure 10 is obtained.

The image-capturing module 34, set alongside the projector module 30within the package of the device for formation of three-dimensionalimages 32, has a respective field of view FOV′ such as to frame thepattern of parallel lines projected by the projector module 30 andgenerate corresponding two-dimensional images.

In a known manner, here not described in detail, the processing module35 is thus able to reconstruct a three-dimensional image of the object37, and of the surrounding environment, exploiting the parallaxprinciple, processing with appropriate algorithms the images captured bythe image-capturing module 34 in order to determine the depth anddistances in the three-dimensional space.

Finally, it is clear that modifications and variations may be made towhat has been described and illustrated herein, without therebydeparting from the scope of the present invention, as defined in theannexed claims.

In particular, the conformation of the cavity 15 may possibly differfrom the one previously illustrated, provided that it is in any caseshaped in such a way that the second body 14 in which it is obtaineddoes not hinder the reflected light beam, as previously discussed indetail.

In this regards, FIG. 11 is a schematic illustration of a possiblevariant embodiment of the mirror micromechanical structure 10.

In this variant, the cavity 15 does not extend as far as the first edgewall 14 a of the mirror micromechanical structure 10, but in any casehas an elongated conformation along the second axis y such that thepresence of the lateral delimitation wall (in this case entirelysurrounding the cavity 15) does not hinder the reflected light beamwithin the desired field of view and with reference to the rotationsallowed for the mobile mass 12. This variant embodiment envisages,however, a greater size of the die of the mirror micromechanicalstructure 10.

Moreover, the cavity 15 could possibly extend from either side of theaxis of rotation A, until it reaches both of the edge walls 14 a, 14 bof the micromechanical structure 10.

Driving of the mobile mass 12 could also be obtained employing adifferent technique, for example a piezoelectric or magnetic technique.

As it has been previously emphasized, the mirror micromechanicalstructure 10 may in general be used for any optical system and portableapparatus that requires reflection of a light beam with a reducedoccupation of space and a wide field of view.

What is claimed is:
 1. An optical device, comprising: a mirrormicromechanical structure having: a mobile mass which carries a mirrorelement and is configured to be driven in rotation for reflecting anincident light beam with a desired angular range; said mobile masssuspended above a cavity provided in a supporting body includingsemiconductor material, wherein said cavity is so shaped that saidsupporting body does not hinder the light beam reflected by said mirrorelement within said desired angular range.
 2. The device according toclaim 1, further comprising: an image-projection module including saidmirror micromechanical structure and a source operable for generatingsaid incident light beam; and an image-capturing module operativelycoupled to said image-projection module for capturing images associatedto the light beam reflected by said mirror micromechanical structure. 3.The device according to claim 2, further comprising a processing moduleconfigured to receive and process the images captured by saidimage-capturing module to form three-dimensional images.
 4. The deviceaccording to claim 2, wherein the image-capturing module has a field ofview which at least partially overlaps the desired angular range.
 5. Thedevice according to claim 4, wherein the image projection module isconfigured to project within the desired angular range a pattern ofparallel lines, and wherein the image-capturing module is configured toobtain corresponding two-dimensional images.
 6. The device according toclaim 5, further comprising a processing module configured to receiveand process the two-dimensional images captured by said image-capturingmodule to reconstruct a three-dimensional image of the an object and itssurrounding environment.
 7. The device according to claim 6, wherein theprocessing module uses a parallax principle processing algorithms toprocess the two-dimensional images captured by the image-capturingmodule in order to determine depth and distance in three-dimensionalspace.
 8. The device according to claim 1, wherein said cavity extendsas far as a first side edge wall of said supporting body, being opentowards, and communicating with, an outside edge of said mirrormicromechanical structure at said first side edge wall.
 9. The structureaccording to claim 8, wherein said mobile mass has a rotational movementabout an axis of rotation, and wherein said cavity extends as far assaid first side edge wall along an axis of extension, transverse to saidaxis of rotation.
 10. The structure according to claim 8, wherein saidcavity has a first width, at said first side edge wall, and a secondwidth, underneath said mobile mass, wherein said second width is smallerthan said first width.
 11. The structure according to claim 1, whereinsaid mobile mass is coupled to anchorages towards said supporting bodyvia elastic torsional elements which define an axis of rotation for saidmobile mass.
 12. The structure according to claim 1, further comprisinga structural body in which said mobile mass is defined and coupled tosaid supporting body.
 13. The structure according to claim 12, whereinsaid structural body and said supporting body are defined from asilicon-on-insulator wafer.
 14. The structure according to claim 1,wherein said mirror element is carried by said mobile mass in acondition of rest at an angle of inclination with respect to saidincident light beam comprised between 40° and 50°, and wherein saidmovement of rotation of said mobile mass for said angular range has anangular extension comprised between −(15 to 25)° and +(15 to 25)° withrespect to said condition of rest.
 15. The structure according to claim1, wherein the supporting body has a top surface including said cavityand wherein said mobile mass has a top surface coplanar with the topsurface of the supporting body and suspended above said cavity in aconfiguration which permits oscillation of the mobile mass about an axisof rotation.
 16. The structure according to claim 15, wherein saidcavity provides a first open region on a first side of the mobile massand a second open region on a second side of the mobile mass, saidsecond side located opposite the first side relative to the axis ofrotation, said second open region being larger than said first openregion.
 17. The structure of claim 16, wherein said supporting body hasa side edge, and wherein said second open region extends to said sideedge.